Acoustics

Principles of Acoustic Emission Testing
Passive acoustic emission detection technology draws heavily from the field of anti-submarine warfare where it has been used for decades in undersea warfare, detecting the telltale sounds of submarines and ships. This same basic technology was adapted to PCCP pipelines, capitalizing on the infallible acoustic sounds made by the broken prestressed reinforcing wire as it releases its energy. In anti-submarine warfare and in pipeline testing, acoustic emissions of interest are detected by a series of hydrophones and screened for the known acoustic signatures that are emitted by an event of interest. The opening of a torpedo door is an event of interest to submariners. The release of energy by the prestressing steel is an event of interest to PCCP owners. These and most other events have unique acoustic properties which allow these events to be distinguished.

Prestressed concrete pipe is reinforced by spirally wrapping high strength wire around a concrete cylinder. If the pipe is in a state of distress, the prestressing wire will be involved. When this occurs, the wire will break in a relatively brittle fashion, with an instant release of the tensile force up to 5,000 kg (11,000 pounds) in that strand of wire.

Much of this energy is in the form of sound energy which propagates through the pipe core and into the column of water within the pipe. The broken ends of the wire are immediately re-anchored in the protective mortar due to friction and the Poisson effect. If the deterioration continues, the protective mortar will be further compromised and the stored energy within the prestressing wire will be released in a series of discrete events. As more wires in the area of distress are involved, they too will break. The process of deterioration leading up to a corrosion-related failure takes several years to run its course, and it is a very noisy process.

The precise identification of the arrival times of these signals at a series of hydrophones is used to locate the source of the events. Sound travels through water at a known and constant speed. That speed is approximately one mile per second, or about five times the velocity of sound in air at sea level. The time it takes for a sound to arrive at a hydrophone is directly related to the distance it travels. The greater the distance, the longer the time. Therefore, the physical location of a wire break can be determined by comparing the arrival times of that event at both hydrophones.

The figure below highlights the components of an acoustic emission detection system. The illustration simulates a wire break close to the left hydrophone. The sound from this event will be detected first at the left hydrophone and momentarily later at the right hydrophone. By comparing the difference in arrival times between the two hydrophones to millisecond accuracy, the location of the event can be determined.

SKETCH OF AET
Development of Acoustic Test Equipment
Early experiments confirmed that breaking prestressing wire does emit a sound with a unique acoustic signature, and that the quantity of data was surprisingly high. As a result, the condition of the pipeline began to come into focus within a few days. It did not require many months of testing as had been anticipated. This is attributed to the relatively noisiness of the pipe deterioration process, and suggested that a permanently installed system may not be necessary. A portable system with an approximate test period of days or weeks was discussed. The technology requires that the precise instant of detection at multiple hydrophones to determine the source of the sound, and this implied a real-time communication with each hydrophone sensor. To improve the efficiency, we needed some method of replacing the buried cable or extensive system of antennas and radio transmitters, in order to be practical.

PTI met this need by developing the proprietary AH-4 Pipeline Test System. This system consists of:

A series of two or more sensitive hydrophones that are used to detect noise in the pipeline. These sensors are mounted on the end of a stainless steel shaft which is inserted into the pipeline through a series of seals and valves while the pipeline is in service at operating pressure. The hydrophones are usually installed in the pipeline through existing air valves after temporarily removing the air handling mechanism.

Signals from these hydrophones are monitored by a small computer located close to the hydrophone. This battery-operated computer screens all acoustic activity against the acoustic signature of prestressing wire-related emissions. The computer records all signals matching the wire signal characteristics on data storage disks for later processing.

A third component of the system, and the key to elimination of the cumbersome telemetry, is the global positioning system (GPS) antenna and processor which is incorporated into the acoustic system. This feature accomplishes two purposes. Primarily it serves as a very accurate clock. It determines the precise time of passage of the signal to an accuracy greater than a thousandth of a second. This precise time of passage is compared to the same information at adjacent hydrophones to determine the point of origin of the sound. Coincidentally it provides the location of the hydrophone in latitude and longitude so that there is no ambiguity as to where a signal was detected.